In invisible radiation systems such as x-ray, gamma ray and other systems utilizing invisible radiation, it is often necessary to employ devices for detecting the invisible radiation and converting it to another form of energy such as electrical signals which may more easily adapted and controlled for subsequent utilization in the systems.
It has been common practice to employ an x-ray tube for producing an x-ray image and a fluoroscopic screen for detecting the x-ray image and converting it into a corresponding visible image. Also common in the art is the use of an image intensifier tube which has a fluorescent screen at one end which detects the x-ray image and converts it to a visible image, a photoemitter which converts the visible image to an electron image, and an output fluorescent screen which converts the electron image to a highly intensified visible image.
Other known invisible radiation detectors include devices based on scintillator crystals combined with photomultiplier tubes. The scintillator crystal functions to convert the radiation to visible light with the light sensitive photomultiplier tubes converting this light to an electrical signal. Suitable scintillator materials include materials such as sodium iodide, calcium fluoride, bismuth germanate or the like.
Another detector device which has been commonly used is an ionization chamber or tube containing a gas which becomes ionized by the invisible radiation with the tube producing electrical signals in response to such ionization.
Semiconductor devices have been made sensitive to invisible radiation such as x-rays or gamma rays by making them of lithium-drifted silicon or germanium so that their P-N junctions would be effectively activated by the penetrating radiation.
None of such prior art invisible radiation detectors is entirely suitable for use in systems such as computerized tomography for various reasons. Fluoroscopic screens and image intensifiers, are obviously unsuitable and do not produce electrical signals. Photomultiplier and ionization tubes are bulky and relatively inefficient. Semiconductor devices must be specially constructed and are relatively inefficient and noisy.